Traditional railroad lines, whether laid on wooden or reinforced concrete sleepers, require periodic maintenance of the ballast, which is an expensive and awkward procedure. Frequently such maintenance must be carried out in difficult environmental conditions, as at bridges or tunnels.
In fact, owing to particular morphological conditions and alignment limitations (gradients of a few units per thousand), both in Italy and most foreign countries, modern railroad lines are largely laid in tunnels or bridges and viaducts.
The need for novel approaches, in laying railroad lines, which can do without the traditional ballast system, is therefore apparent.
Known systems are being tested or used on various national railroad lines, which provide for the use of platforms for transferring the loads to the underlying ground and providing a rigid support for the rails. In particular, a series of systems provide for the utilization of concrete platforms cast in the field, reinforced with slack steel reinforcement wherein pre-stressed reinforced concrete sleepers are embedded which support the rails. That approach, while affording a number of advantages, brings about some problems the most important of which are the impossibility of reacting in an effective way to sagging due to settling of the underlying ground. A problem of durability stems from the fact that the concrete cast in situ, even if of excellent quality and prepared with adequate additives, when subjected to high dynamic loads and especially to repeated temperature changes, soon becomes cracked which results in deterioration of the concrete due to water infiltration.
This cracked state is specially objectionable at the contact sections between pre-fabricated sleepers and the cast work, where owing to the mechanical and rheological conditions of the two different concrete types, cracking and separation phenomena can readily develop.
Another system, in use with Japanese railroad lines, provides for the platform to be formed of pre-fabricated elements laid on a continuous foundation of concrete constituting the bottom of a tunnel or the scaffolding of a bridge, with the interposition of a bitumen mortar layer and possibly a rubber layer. The bond between the pre-fabricated elements and the foundation is provided by a concrete cylinder (stopper) integrated with the foundation by casting in situ and protruding therefrom. This system also has, however, disadvantages the most important of which is the rapid rate of deterioration of the weakest parts of the system, in particular of the bitumen mortar and rubber which, at the joint between the pre-fabricated element and the stopper, is left exposed to weather and temperature changes, as well as to the aggressive agents present along railroad lines.